1. Field of the Invention
The present invention relates to a one-dimensional image sensor for receiving light reflected from an original copy on which the light has been irradiated and for converting the received light into electric signals. More specifically, the present invention is directed to an image reading apparatus such as facsimile machines and image scanners to which the one-dimensional image sensor according to the present invention is applied.
2. Description of the Related Art
FIG. 2 is a circuit diagram of a contact-type one-dimensional image sensor IC used in conventional image reading apparatuses. There is shown in FIG. 2 a system for successively turning ON switches S1-Sn to read out outputs of phototransistors P1-Pn on a common signal line. The above-mentioned contact-type phototransistor one-dimensional image sensor is described in Japanese Patent Application Laid-Open No. Sho 61-124171.
In such a one-dimensional image sensor, however, there is a problem in that since carriers remaining on the bases of the phototransistors are removed through an emitter, a base potential cannot be sufficiently reset to an initial state, resulting in increased residual images.
In view of the above, an object of the present invention is to provide an image sensor having diminished residual images with a low cost.
In order to solve such a problem inherent in conventional image sensors, according to the present invention, a photodiode is used for a light receiving element, and an output terminal of a photodiode is reset to a constant voltage to thereby realize reduced residual images. Further, an image signal output of the photodiode and a reference voltage output immediately after the photodiode is reset are succeedingly performed using a clamping circuit. Therefore, an image sensor having a small fluctuation in dark outputs and a good S/N ratio can be obtained at a low cost.
According to the present invention, there is provided an image sensor so arranged as follows.
In an image sensor for successively reading out outputs of a plurality of photodiodes as image signal outputs on a common signal line via a first amplifier, an operation is repeated in which an image signal output of a first photodiode is read out, and thereafter an output terminal of this photodiode is reset to a constant voltage; and after termination of the reset, an output of this photodiode is again read out as a reference voltage output; and then an image signal output of a second photodiode is read out.
Further, the image sensor according to the present invention is arranged so that an output of the common signal line is supplied to a second amplifier, whereby the image signal output and the reference voltage output are amplified.
In addition, arrangement is made in such a manner that a capacitance is connected in series between the output of the second amplifier and the output terminal of the image sensor to form a clamping circuit in which the output terminal is connected to a reset switch. The reset switch is kept ON during the time interval between the time immediately before the image signal output of each photodiode initiates and the time immediately before the image signal output of each photodiode terminates, and a potential of the output terminal is thus held to the reset voltage.
With the foregoing arrangement, if a sample and hold operation is carried out immediately before the reference voltage output of each photodiode terminates to be read out, an image sensor having a small fluctuation in dark output between the respective bits and a good S/N ratio can be obtained. That is, since the image signal output and the reference voltage output of the respective bits pass through the same output path, the offset due to switching noise or the amplifiers is completely carried out through the clamping circuit.
Also, each photodiode is reset to a constant voltage, and thus an image sensor having reduced residual images can be realized.